In an unbalanced output, a three-pole terminal with a diameter of 3.5 mm is used, and an audio signal is transmitted by two kinds of “hot” and “cold”. Meanwhile, in a balanced output, a four-pole terminal with a diameter of 2.5 mm is used, and an audio signal is transmitted by two kinds of “positive” and “negative” (for example, see JP 2013-005291 A). “Negative” is opposite phase of “positive”. By inverting phase of “negative” and mixing inverted “negative” signal with “positive” signal, amplitude of audio signal becomes twice. Further, ground potential is stable, and sound quality can be improved because output current does not flow to ground.
An applicant files JP 2015-171945 in Japan. FIG. 3 is a diagram illustrating a circuit configuration of a digital audio player according to JP 2015-171945. As illustrated in FIG. 3, the digital audio player includes DACs (D/A converters) 107 and 108, amplification circuits 109 and 110, a DC/DC converter 113, linear regulators 114 and 115, a balanced output jack 116 and an unbalanced output jack 117.
The DC/DC converter 113 boosts voltage which is supplied from a battery and supplies boosted voltage to the linear regulators 114 and 115. The linear regulator 114 supplies power supply voltage to the DAC 107 and the amplification circuit 109. The linear regulator 115 supplies power supply voltage to the DAC 108 and the amplification circuit 110.
LR 2 channel digital audio data of I2S system is input to the positive side DAC 107 (for a non-inverted signal). The DAC 107 D/A-converts the LR 2 channel digital audio data into LR 2 channel analog audio data. The positive side amplification circuit 109 amplifies the LR 2 channel analog audio data which is D/A converted by the DAC 107. The amplification circuit 109 includes two amplifiers 118 and 119. The amplifier 118 amplifies L channel analog audio data. The amplifier 119 amplifies R channel analog audio data. The analog audio data which is amplified by the amplification circuit 109 is output to the balanced output jack 116 and the unbalanced output jack 117.
The LR 2 channel digital audio data of I2S system is input to the negative side DAC 108 (for an inverted signal). The DAC 108 D/A-converts the LR 2 channel digital audio data into the LR 2 channel analog audio data. LR 2 channel inverted analog audio data that the LR 2 channel analog audio data which is D/A-converted by the DAC 108 is inverted is input to the negative side amplification circuit 110. The amplification circuit 110 amplifies the LR 2 channel inverted analog audio data. The amplification circuit 110 includes two amplifiers 120 and 121. The amplifier 120 amplifies L channel inverted analog audio data. The amplifier 121 amplifies R channel inverted analog audio data. The inverted analog audio data which is amplified by the amplification circuit 110 is output to the balanced output jack 116.
In case of the balanced output, an SoC (System on a Chip) supplies power supply voltage from the linear regulator 114 to the DAC 107 and the amplification circuit 109. Further, the SoC supplies power supply voltage from the linear regulator 115 to the DAC 108 and the amplification circuit 109. In case of the unbalanced output, the SoC supplies power supply voltage from the linear regulator 114 to the DAC 107 and the amplification circuit 109. Meanwhile, the SoC shuts down the linear regulator 115. For this reason, the linear regulator 115, the DAC 108, and the amplification circuit 110 shut down.
In case that the circuit which is illustrated in FIG. 3 is applied to a mobile phone, when headphones are connected to the balanced output jack and the unbalanced output jack, if call arrives, a problem occurs. This is because a positive side circuit is used to the balanced output and the unbalanced output commonly. FIG. 4 is a diagram for describing a problem which occurs in case where the circuit which is illustrated in FIG. 3 is applied to the mobile phone. In case of the unbalanced output, as illustrated in FIG. 4, the negative side amplification circuit 110 is an OFF state. In this state, if current is output from the positive side amplification circuit 109, balanced load current flows to the negative side amplification circuit 110 which becomes OFF, and it is possible that the negative side amplification circuit 110 damages. In case that the headphones are connected to the balanced output jack and the unbalanced output jack, it is thought that an audio is output from the speaker. However, it is normal that the audio is output from the headphones because the headphones are connected.